Grayscale shading can be generated on a screen of an analog display such as a cathode ray tube (CRT) by varying the brightness control voltage at the control input to the analog display. The analog display uses this varying voltage to modulate the brightness of each pixel and thus produce the grayscale level. Unfortunately, this same grayscale shading technique does not lend itself to digitally commanded displays such as multiplexed liquid crystal displays (LCDs), light emitting diode (LED) displays, electroluminescent (EL) displays, field emission displays (FEDs), or plasma displays wherein individual pixels (discrete light source regions including emissive, transmissive, and reflective types) can be commanded to switch towards only one of two brightness levels, ON or OFF (i.e. white or black). Such digital displays generally lack an analog control and therefore do not have a direct means independent of their power lines for commanding a pixel towards an intermediate brightness level (grayscale) between black and white.
Multiplexed displays typically have only two electrodes provided at each pixel area for addressing a pixel area and energizing the pixel area to either produce the appearance of a fully lit (white) pixel or to produce the appearance of a fully darkened (black) pixel. Since an analog means for controlling brightness level is not available on many types of digital displays, alternative digital techniques have been proposed for giving a viewer the perception of grayscale shading.
One of the proposed alternative techniques is a so-called "pulse-width modulation" scheme wherein the width of pixel energizing pulses is modulated between wide and narrow values to create a grayscale effect.
There have been proposed several methods for providing the gradation of the display brightness using pulse-width modulation schemes, such as U.S. Pat. No. 4,006,298, Japanese article "TV Display on an AC plasma Panel" by K. Takikawa, or Japanese Patent Publication 51-32051 or Hei2-291597, wherein a single frame period of a picture to be displayed is divided with time into multiple subframes (G1, G2, G3, etc.) each of which has a specific time length for lighting a cell so that the visual brightness of the cell is weighted. This method is illustrated in FIG. 1 wherein pixels on a single horizontal line are selectively written and illuminated for a specific length of time, pixels on the next horizontal line are then written and display for the specific length of time, etc. until all lines have been written and displayed. Gradation of visual brightness is proportional to the length of time that the pixel is illuminated during the frame. Therefore, different time lengths are allocated to the subframes such that the gradation is determined by an accumulation of display time in the selectively operated subframes.
One problem of this method is in that the second subframe must wait until the completion of the first subframe for all lines to be written thus creating an idle period for each line. This idle time has the effect of diluting the gradation technique by introducing additional off time that precludes the use of a full white (100% gradation level) pixel. To minimize the idle time, high frequency writing and drive circuits are required which results in increased power consumption and usually less operating margin.
A second method of "pulse-width modulation" has been proposed in U.S. Pat. Nos. 4,559,535; 5,187,578 and 5,541,618 wherein a single frame period of a picture to be displayed is divided with time into multiple subframes (G1, G2, G3, etc.) each of which has a specific time length for lighting a cell so that the visual brightness of the cell is weighted. This method is illustrated in FIG. 2 wherein all pixels in the display are written with one addressing pulse and then pixels are selectively erased based on the grayscale value for that subframe. Illuminated pixels are displayed for the specific length of time and then erased prior to activating the next subframe. This method eliminates the idle time previously described and has the further advantage of "priming" all pixels before they are displayed, if this is important in the technology. Thus it removes any time effects that may occur as the image changes since there are no time gradients produced that may become visible to the eye.
A third method involves an ordered dither arrangement such as described in U.S. Pat. No. 3,937,878 wherein grayscale levels are displayed as a distribution of pixels whose spatial density is ordered such that the distribution represents the amount of light emanating from a specific location of the display. The technique may be enhanced by applying hysteresis methods well known in the art to the incoming signal such that the distribution (grayscale value) for the area is only changed when a significant change in the signal occurs. This technique avoids the small changes in grayscale values that usually occurs in digitizing an analog signal. Other space distribution methods of displaying grayscale values have been reported such as described in U.S. Patent No. 5,185,002.
A problem with all of the aforementioned digital techniques is the occurrence of flickering, surface streaming, line crawl, contouring, and/or color change artifacts. The article by Takikawa, cited above, described these disturbances and their cause (but incompletely) as early as 1977. In brief, these artifacts are due to the ability of the human eye to preferentially detect motion and patterns. An appreciation of this aspect can be gained from the physiochemistry and construction of the eye and optic nerve path to the brain, such as described in the Feynman Lectures on Physics, volume I, pp. 35-1 and 2. The interesting thing is that in the retina of our eye, each of the cells which is sensitive to light is not connected by a fiber directly to the optic nerve, but is connected to many other cells, which are themselves connected to each other. There are several kinds of cells; there are cells that carry the information toward the optic nerve, but there are others that are mainly interconnected "horizontally." The main thing is that the light signal is already being "thought about" before it reaches to the brain. That is to say, the information from the various cells does not immediately go to the brain, spot for spot, but in the retina a certain amount of the information has already been digested, by a combining of the information from several visual receptors. It is therefore understood that some brain-function phenomena occurrs in the eye itself Thus, the eye is sensitive to patterns and motion as well as the viewing of a pretty scene.
The time/space relationship of digital pulses in display systems leads to these psychovisual phenomenon. The eye and brain perceives certain pulsing patterns of a digital image as having unexpected patterns or moving portions. To be sure, such artifacts are, to some extent, familiar even on film movies and TV CRT display systems which are all basically digitalized in time. TV's flicker badly and have obvious interlace separation with moving images. Home film movies were good examples of flicker and jitter, and wagon wheels "appear" to go backwards even in the best movie theaters. Such "false image artifacts" can become more severe in display images which are digitized in both time and space. In this case contour streaming, false colors, and false movements as well as flicker many also be perceived.
Such digital image artifacts are well known in the display industry and various methods have been devised to mitigate or minimize them. Such techniques include adding "leveling" pulses such as in U.S. Pat. No. 5,430,458 and as described in the literature, for example 1997 SID Symposium Digest paper 19.1 "Performance Features of a 42 in. Diagonal Color Plasma Display, T. Hirose, et. al. Other techniques involve image preprocessing to detect motion and in certain cases eliminate frames in order to achieve images more pleasing to the eye. For example, U.S. Pat. No. 4,602,273 describes a display with image filters to avoid line-crawl artifacts in particular.